It begins as nothing but warm water and a scatter of thunderstorms. Nine stages later it has an eye. Here is every step of the sequence, in order.
A hurricane needs sea water at 26.5C (80F) — and not only at the surface. That warmth must reach about 50 metres down, or the storm churns up cold water and stalls.
Warm water evaporates and that moist air rises. As it lifts, it leaves less air behind at the surface, so pressure drops — and surrounding air rushes in to fill the gap.
The rising air condenses into tall thunderstorms. Once a loose group of them starts sharing a single low-pressure centre, forecasters call it a tropical disturbance.
Air rushing inward is deflected by Earth's rotation, so the cluster starts to turn. It is also why storms never form within about 500 km of the equator — there the effect vanishes.
The storm has to stack straight upward. If high-altitude winds blow far faster than surface winds, that wind shear tilts the tower and tears the young system apart.
When the spiral tightens and sustained winds reach 39 mph, it becomes a tropical storm and is given a name. The Atlantic's first two this year were Arthur and Bertha.
At 74 mph it is a hurricane. Air sinking at the centre clears a calm eye, ringed by the eyewall — the band carrying the fiercest wind and rain in the whole storm.
Over very warm water with no shear, a hurricane can gain 35 mph in 24 hours — rapid intensification. It is how a modest storm turns dangerous just before it arrives.
Wind snaps the trees, but the ocean does the worse damage. Storm surge shoves sea water inland ahead of the eye, and it is usually the greatest threat to life on the coast.
Over land the warm-water supply is cut off and the engine winds down within days. Every hurricane dies by losing the one ingredient that built it in the first place.